Devices and methods for visually indicating the alignment of a transcutaneous energy transfer device over an implanted medical device

ABSTRACT

The present disclosure involves a charging system for charging an implanted medical system. The charging device includes a replenishable power supply. The charging device includes a coil assembly electrically coupled to the power supply. The coil assembly includes a primary coil and a plurality of sense coils positioned proximate to the primary coil. The charging device includes electrical circuitry operable to: measure an electrical parameter of the coil assembly; and determine a position of the coil assembly relative to a position of the implanted medical device based on the measured electrical parameter. The charging device includes a visual communications interface operable to: receive an input from the electrical circuitry; and visually display on a screen the position of the coil assembly relative to the position of the implanted medical device based on the input received from the electrical circuitry.

PRIORITY DATA

The present application is a continuation application of U.S. patentapplication Ser. No. 14/249,425, filed on Apr. 10, 2014, now U.S. Pat.No. 9,031,666 issued May 12, 2015, which is a continuation applicationof U.S. patent application Ser. No. 13/185,636, filed on Jul. 19, 2011,now U.S. Pat. No. 8,700,175 issued Apr. 15, 2014, the disclosures ofeach are hereby incorporated by reference in their entirety.

BACKGROUND

Implantable medical devices for producing a therapeutic result in apatient are well known. The implanted medical device often requireselectrical power to perform its therapeutic function. This electricalpower is derived from a power source. There are many kinds of powered,implantable medical devices that are powered by an external powersource. It is recognized that other implantable medical devices areenvisioned which also utilize energy transferred from or delivered by anexternal source.

By way of example, one type of powered, implantable medical device is aneurostimulation device. Neurostimulator devices, such as implantablepulse generators (hereinafter “IPGs”), are battery-powered devices thatdeliver therapy in the form of electrical stimulation pulses to treatsymptoms and conditions, such as chronic pain, urinary incontinence,Parkinson's disease, deafness, or epilepsy, for example. IPGs deliverneurostimulation therapy via leads that include electrodes locatedproximate to the muscles and nerves of a patient. Treatments may requiretwo external devices: a neurostimulator controller and aneurostimulation device charger. Neurostimulator controllers arefrequently used to adjust treatment parameters, select programs, anddownload/upload treatment information into/from the implantable device.Neurostimulation device chargers are used to transcutaneously rechargebatteries or capacitors in the implanted device.

Transcutaneous transmission of energy from an external transmitter to aninternal receiver is known in the prior art. Several implantable medicaldevices, including an IPG, employ a replenishable power source such as astorage capacitor or a rechargeable battery. This replenishable powersource can be recharged when necessary using transcutaneous energytransfer (hereinafter “TET”) from an external power source, i.e., energyis transferred non-invasively through the skin via electromagneticcommunication between an external transmitter coil and an implantedreceiver coil. TET involves the process of inductive coupling betweentwo coils positioned in close proximity to each other on opposite sidesof a cutaneous boundary. The external transmitter coil, composed of aplurality of wire windings, is energized by a source of alternatingelectrical current. This flow of electrical current in the externaltransmitter coil induces a corresponding current in the windings of theinternal receiver coil. This resultant current can be applied torecharge the battery of the implanted medical device, or, in addition,can directly energize the IPG. Optimum transcutaneous energy transferefficiency is achieved when the external transmitter coil is disposed onthe patient's skin, directly opposite the implanted receiver coil, witha minimum separation distance between the external transmitter coil andthe implanted receiver coil.

Though TET provides the advantage of non-invasive recharging of an IPG,TET is not without certain shortcomings. For example, the efficiency oftranscutaneously inducing a current in the implanted coil isdetrimentally affected if the external TET coil and implanted coil arenot properly aligned. Though the operator of the TET device may use thevisual or tactile signs of implantation to approximate the location ofthe IPG, precise alignment of the TET coil and charging coil isextremely difficult without the aide of an alignment indicator. Becausethere is no physical connection between the external TET device and theIPG to provide feedback, ascertaining whether the efficiency of energytransfer is maximized is problematic.

Even if the TET device is properly aligned with the IPG at theinitiation of the charging process, the correct alignment of the devicesmay not endure over the period of time required for energy transfer.Energy transfer can continue for a significant period of time, rangingfrom several minutes to hours, before the IPG is fully recharged. Duringthis time, it is often impracticable for the external TET coil tomaintain ideal alignment with the IPG receiver coil. The patient'smovement may cause the external TET coil to move, thereby misaligningthe TET coil with the IPG receiver coil and reducing the efficiency ofenergy transfer. Therefore, it would be advantageous to provide a TETdevice that could indicate the real time alignment (or misalignment) ofthe devices and visually direct the operator toward regaining optimalalignment, thereby increasing the efficiency of energy transfer anddecreasing the amount of time required for the IPG charging process.

In addition, prolonged exposure to the electromagnetic fields and heatgenerated by the external TET coil and the IPG can result in damage tohuman skin and adjacent tissues. The resulting damage generallyincreases with the length of exposure time. Therefore, it is desirableto limit the amount of time required to recharge the battery of an IPGusing a TET device. If the devices are poorly aligned, the efficiency oftranscutaneous energy transfer is reduced and the length of timerequired to charge the IPG is increased, thus extending the patient'sexposure time to electromagnetic radiation and heat. Reducing theexposure time by improving device alignment would reduce potentialtissue injury. Therefore, though existing TET devices have beengenerally adequate for their intended purposes, they are not entirelysatisfactory in every aspect.

SUMMARY

One of the broader forms of the present disclosure involves a chargingdevice for charging an implanted medical device. The charging deviceincludes a replenishable power supply; a coil assembly electricallycoupled to the power supply, the coil assembly including a primary coiland a plurality of sense coils positioned proximate to the primary coil;electrical circuitry operable to measure an electrical parameter of thecoil assembly and determine a position of the coil assembly relative tothe implanted medical device based on the measured electrical parameter;and a communications interface operable to convey the position of thecoil assembly relative to the implanted medical device.

In an embodiment, the communications interface includes a display unitthat is operable to visually display the position of the coil assemblyrelative to the implanted medical device.

In an embodiment, the communications interface is operable to givedirectional instructions to a user to align the coil assembly with theimplanted medical device.

In an embodiment, the power supply, the electrical circuitry, and thecommunications interface are integrated into a charger unit; and thecharger unit is electrically coupled to the coil assembly through aconductive cable.

In an embodiment, the sense coils are located in different layers of asubstrate and are aligned with one another.

In an embodiment, at least two of the sense coils are perpendicularlyoriented with respect to each other.

In an embodiment, the primary coil is located over the sense coils.

In an embodiment, the measured electrical parameter includes anelectrical voltage of at least one of the sense coils.

In an embodiment, the electrical circuitry is operable to translate theelectrical parameter into a positional displacement of the coil assemblyrelative to the implanted medical device.

In an embodiment, the electrical circuitry contains: an amplifiersection that is coupled to at least one of the sense coils; a rectifiersection that is coupled to the amplifier section; a filter section thatis coupled to the rectifier section; and a comparators section that iscoupled to the filter section.

Another one of the broader forms of the present disclosure involves amedical charging system. The charging system includes a coil assemblystructure that includes: a plurality of position-sensing coils; and aprimary coil disposed proximate to the position-sensing coils; circuitrythat is electrically coupled to the coil assembly structure, thecircuitry being configured to: make measurements of electricalcharacteristics of the position-sensing coils; and translate themeasurements into a location displacement of the coil assemblystructure; and a visual display configured to display the location ofthe coil assembly.

In an embodiment, the medical charging system further includes a medicaldevice implanted in a body tissue, and the circuitry that iselectrically coupled to the coil assembly structure is configured totranslate the electrical characteristics of the position-sensing coilsinto the location displacement of the coil assembly structure withrespect to the medical device.

In an embodiment, the medical device contains a coil, and the circuitryis configured to translate the electrical characteristics of theposition-sensing coils into relative alignment between the coil assemblystructure and the coil of the medical device.

In an embodiment, the visual display is configured to display a locationof the medical device.

In an embodiment, the visual display is configured to give directionalmovement instructions for bringing the coil assembly structure intoalignment with the medical device.

In an embodiment, the electrical characteristics are induced voltages atthe position-sensing coils in response to a magnetic flux.

In an embodiment, the circuitry is configured to translate a voltage ineach position-sensing coil into a respective locational displacement ofthe coil assembly structure in one of a plurality of differentdirections.

In an embodiment, the circuitry is configured to detect a zero crossingevent.

In an embodiment, the position-sensing coils have different orientationsand are substantially concentrically located.

In an embodiment, the medical charging system further includes a powersource that is electrically coupled to the coil assembly structure.

In an embodiment, the circuitry and the visual display are implementedinside a handheld control device, and the handheld control device iselectrically coupled to the coil assembly structure through a conductivecable.

One more of the broader forms of the present disclosure involves anapparatus for charging a medical device implanted in a body tissue. Theapparatus includes: coil assembly means for transcutaneously deliveringelectrical power to the implanted medical device, wherein the coilassembly means includes a primary coil and first and secondposition-sensing coils located adjacent to the primary coil; circuitrymeans for gathering electrical data associated with the coil assemblymeans and for obtaining locational information of the coil assemblymeans relative to the implanted medical device based on the gatheredelectrical data; and communication means for communicating thelocational information of the coil assembly means and for givingdirectional instructions based on the locational information.

In an embodiment, the communication means includes visual display meansfor visually indicating an extent of alignment between the coil assemblyand the implanted medical device.

In an embodiment, the communication means includes means for instructinga user on how to move the coil assembly means in order to achievealignment between the coil assembly means and the implanted medicaldevice.

In an embodiment, the electrical data includes voltages in the first andsecond position-sensing coils, and the circuitry means includes meansfor translating the voltages into physical displacements of the coilassembly means in different first and second directions, respectively.

In an embodiment, the circuitry means includes: amplifying means foramplifying a signal received from at least one of the first and secondposition-sensing coils; rectifying means for converting an alternatingcurrent (AC) portion of an output from the amplifying means to a directcurrent (DC) portion; filtering means for filtering an output of therectifying means; and comparator means for detecting a zero-crossingevent.

In an embodiment, the apparatus further includes a power supply meansfor supplying power to the coil assembly means.

In an embodiment, the apparatus further includes: housing means forhousing the power supply means, the circuitry means, and thecommunication means; and cabling means for coupling the housing meanswith the coil assembly means.

Yet another one of the broader forms of the present disclosure involvesa method for charging a medical device implanted in a body tissue. Themethod includes: providing a coil assembly, the coil assembly includinga primary coil and a plurality of position-sensing coils; measuring anelectrical parameter of the coil assembly; determining, in response tothe measuring, positional information of the coil assembly relative tothe implanted medical device; and communicating the positionalinformation.

In an embodiment, the providing is carried out in a manner such that:the primary coil is located over the position-sensing coils; and theposition-sensing coils are concentrically-located and have differentorientations.

In an embodiment, the measuring includes measuring a first voltage at afirst one of the position-sensing coils and measuring a second voltageat a second one of the position-sensing coils.

In an embodiment, the determining includes: translating the firstvoltage into a displacement of the coil assembly along a first axis; andtranslating the second voltage into a displacement of the coil assemblyalong a second axis different from the first axis.

In an embodiment, the communicating includes visually displayingpositional information of the coil assembly and positional informationof the implanted medical device.

In an embodiment, the communicating includes giving directionalinstructions for moving the coil assembly to achieve alignment with theimplanted medical device.

In an embodiment, the method further includes transcutaneously chargingthe implanted medical device through the coil assembly.

Further aspects, forms, embodiments, objects, features, benefits, andadvantages of the present disclosure shall become apparent from thedetailed drawings and descriptions provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion. In addition, the present disclosuremay repeat reference numerals and/or letters in the various examples.This repetition is for the purpose of simplicity and clarity and doesnot in itself dictate a relationship between the various embodimentsand/or configurations discussed.

FIG. 1 is a diagrammatic view of a neurostimulator device.

FIG. 2 is a diagrammatic view of an external power charging device.

FIG. 3 is a partial cross-sectional side view of a transcutaneous energytransfer (hereinafter “TET”) device, a coil assembly positioned slightlyabove the skin and an implantable pulse generator implantedsubcutaneously.

FIGS. 4 and 5 are top views of a TET device positioned slightly abovethe skin and an implantable pulse generator implanted subcutaneously.FIG. 4 illustrates the TET device being moved in a plane parallel to theskin surface towards the implantable pulse generator. FIG. 5 illustratesthe TET device aligned with the implantable pulse generator.

FIGS. 6A-6B are cut-away top views of a coil assembly comprising atranscutaneous energy transfer coil and positioning-sensing coils.

FIG. 6C is a cross-sectional view of the transcutaneous energy transfercoil of FIGS. 6A-6B.

FIGS. 7A and 7B are graphical representations of the output voltage ofthe synchronous rectifier vs. the transcutaneous energy transfer coildisplacement. FIG. 7A represents the voltage relative to thedisplacement in the X-axis, and FIG. 7B represents the voltage relativeto the displacement in the Y-axis.

FIG. 8 is a schematic block diagram illustrating the electricalcircuitry of one of the position-sensing circuits within the TET device.

FIGS. 9A and 9B are top views of a TET device, showing the displayscreen. FIG. 9A illustrates the display when the TET device and theimplantable medical device are not in alignment, and FIG. 9B illustratesthe display when the TET device and the implantable medical device arein alignment.

FIGS. 10-11 together depict a flow chart illustrating the alignment andcharging of an implantable medical device through the use of a TETdevice.

FIG. 12 is a flowchart illustrating a method of operation of the TETdevice.

FIGS. 13A and 13B are side and posterior views of a human spine,respectively.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will now be made to the embodiments, orexamples, illustrated in the drawings and specific language will be usedto describe the same. It will nevertheless be understood that nolimitation of the scope of the present disclosure is thereby intended.Any alterations and further modifications in the described embodiments,and any further applications of the principles of the present disclosureas described herein are contemplated as would normally occur to oneskilled in the art to which the present disclosure relates.

FIG. 1 is a simplified diagrammatic view of an embodiment of aneurostimulator device 2. The neurostimulator device 2 includes anantenna 3 and a transceiver 4 coupled to the antenna 3. The antenna 3 iscapable of sending signals to an external device and receiving signalsfrom the external device. The transceiver 4 contains transmittercircuitry and receiver circuitry that together carry out bidirectionaldigital communication with the external device. In an embodiment, thesignals are transmitted and received at Radio Frequencies (RF).

The neurostimulator device 2 includes a microcontroller 5 that iscoupled to the transceiver 4. The microcontroller 5 runs firmware 6,which is a control program, to operate control logic 7. The firmware 6includes dedicated low-level software code that is written for aspecific device, in this case the control logic 7. The control logic 7includes digital circuitry that is implemented using a plurality oftransistors, for example Field Effect Transistors (FETs). In theembodiment shown in FIG. 1, the firmware 6 and the control logic 7 areintegrated into the microcontroller 5. In alternative embodiments, thefirmware 6 or the control logic 7 may be implemented separately from themicrocontroller 5.

The neurostimulator device 2 includes stimulation circuitry 8 thatreceives the output of the microcontroller 5. In an embodiment, thestimulation circuitry 8 is implemented on an Application SpecificIntegrated Circuit (ASIC) chip. The stimulation circuitry 8 includeselectrical pulse generation circuitry. Based on the output of themicrocontroller 5, the electrical pulse generation circuitry generateselectrical pulses to a target tissue area.

The neurostimulator device 2 also includes protection circuitry 9 thatis coupled to the output of the stimulation circuitry 8. In anembodiment, the protection circuitry 9 includes direct-current (DC)blocking capacitors and other electrical transient suppressioncomponents. The protection circuitry 9 protects the patient's tissuefrom unwanted electrical signals. The protection circuitry 9 alsoprotects the neurostimulator device 2 from undesirable externalelectrical signals that may be generated by events such as electrostaticdischarge, defibrillation, or electrocautery.

The neurostimulator device 2 may also include a power source 10 andpower circuitry 11. In an embodiment, the power source 10 includes abattery. In another embodiment, the power source 10 includes a coil thatis a part of a transformer (not illustrated). In that case, thetransformer has a charging coil that is external to the neurostimulatordevice 2 and inductively coupled to the coil of the power source 10. Thepower source 10 therefore obtains energy from such inductive coupling tothe charging coil. In some embodiments, the power source 10 may alsoinclude both a battery and a coil. The charging process may be doneusing an external transcutaneous charger, which will be discussed laterin more detail.

The power source 10 provides electrical power to the power circuitry 11.The power circuitry 11 is coupled to the transceiver 4, themicrocontroller 5, and the stimulation circuitry 8. The power circuitry11 supplies and regulates power to these coupled circuitries. In anembodiment, the power circuitry 11 is implemented on an ASIC device.

In an embodiment, the antenna 3, the transceiver 4, the microcontroller5, the stimulation circuitry 8, the protection circuitry 9, the powersource 10, and the power circuitry 11 are all contained within ahermetically-sealed housing or can 15, which may also be considered apart of the neurostimulator device 2. The housing 15 may be made fromtitanium or another suitable durable and/or conductive material that iscompatible with human implantation.

A plurality of conductors (also referred to as lead wires) 20-23 runfrom the internal circuitry through hermetic feedthroughs to one or moreconnectors mounted on the hermetic enclosure 15. The lead wires 20-23plug into, and are removable from, those connectors. In anotherembodiment, the connectors are eliminated, and the lead wires 20-23 aredirectly and permanently connected to the hermetic feedthroughs. In someembodiments, the neurostimulator incorporates the electrode contactsinto its outer surface. In such embodiments, the hermetic feedthroughsmay be designed to incorporate an electrode contact in the tissue-facingside of each feedthrough, or may be designed to have insulated leadwires built into the neurostimulator housing, exterior to thehermetically-sealed enclosure 15, that carry signals between thehermetic feedthroughs and the electrode contacts. It is understood thatthe lead wires 20-23 are shown here merely as examples, and that analternative number of lead wires may be implemented, for example 16 or24 lead wires.

Electrode contacts 30-33 (also referred to as electrodes) are coupled tothe lead wires 20-23. The electrode contacts 30-33 are implanted indifferent areas of a patient's body, where electrical stimulation isdesired. These different areas may be within a few inches of oneanother. In an embodiment, an exterior portion of the housing 15 is alsoused as an electrode contact. In another embodiment, one or moreelectrode contacts can be incorporated into the design of anon-conductive housing 15. In any case, the electrode contacts may alsobe considered parts of the neurostimulator system.

In an embodiment, the neurostimulator device 2 is implemented as an IPGhaving all the components shown in FIG. 1 that is surgically implantedinside the patient's body. Outside the body, the neurostimulator device2 can be programmed using a Clinician Programmer (not illustrated) or aPatient Programmer (not illustrated). The Clinician Programmer is usedby medical personnel to configure the neurostimulator device 2 for theparticular patient and to define the particular electrical stimulationtherapy to be delivered to the target area of the patient's body. ThePatient Programmer is used by the patient himself to control theoperation of the neurostimulator device 2. For example, the patient canalter one or more parameters of the electrical stimulation therapy,depending on the programming and the configuration of theneurostimulator device 2 as set by the Clinician Programmer.

In alternative embodiments, the neurostimulator device 2 can beimplemented as an External Pulse Generator (EPG). In that case, only aportion of the neurostimulator system (for example the electrodecontacts 30-33 and/or portions of the lead wires 20-23) is implantedinside the patient's body, while the neurostimulator device 2 remainsoutside the body. Other than their exact placements, the functionalitiesand the operations of the IPG and the EPG are similar. Thus, in thefollowing discussions, IPG may be used to refer to both an IPG and anEPG. A medical device manufacturer may manufacture and provide theneurostimulator device 2 to a clinician or a patient. Clinicians mayalso provide the neurostimulator device to a patient. Some of thefunctionalities of the microcontroller 5 may be pre-programmed by themanufacturer or may be programmed by the clinician or patient.

The neurostimulator device 2 is capable of varying the amount ofelectrical stimulation delivered to each of the electrode contacts30-33. This is carried out by creating individually controllableelectrical paths, or channels. Each channel includes one of theelectrode contacts 30-33, one of the lead wires 20-23 coupled to theelectrode contact, and respective portions of the protection circuitry 9and respective portions of the stimulation circuitry 8.

As discussed above, an external charging device may be used to provideelectrical power transcutaneously to the power source 10. FIG. 2 is asimplified diagrammatic view of such external charging device 40. Theexternal charging device 40 (also referred to as an external devicecharger or a transcutaneous energy transfer device) includes a powersource 42, a power amplifier 43, a coil assembly structure 44,position-sensing circuitry 46, processor and additional circuitry 47,and a communication interface 48. The power source 42 includes areplenishable power supply, for example a rechargeable battery or areplaceable battery. The output of the power source 42 is amplified bythe power amplifier 43. The power source 42 (in conjunction with thepower amplifier 43) delivers electrical power or electrical energy tothe coil assembly structure 44, the position-sensing circuitry 46, theprocessor and additional circuitry 47, and the communication interface48.

In the illustrated embodiment, the power source 42, the power amplifier43, the position-sensing circuitry 46, the processor and additionalcircuitry 47, and the communication interface 48 are housed within thecharger device 40. The coil assembly structure 44 may be implementedphysically separate from the charger device 40, as is shown in FIG. 2.In such embodiments, the coil assembly structure 44 may be coupled tothe charger 40 through a conductive cable. In alternative embodiments,however, the coil assembly structure 44 may be implemented inside thecharger 40.

In an embodiment, the coil assembly structure 44 includes a core (e.g.,a toroid ferrite core), a primary coil that circumferentially surroundsthe core, and two secondary position-sensing coils that are disposedover the core and the primary coil. It is understood that alternativeembodiments of the coil assembly structure may not have a core. In anembodiment, the primary coil is a multi-layered spiral coil. In anembodiment, the two secondary position-sensing coils each have adouble-D shape, are substantially identical in size, are substantiallyconcentrically disposed over one another, and have substantiallyperpendicular orientations. The coil assembly structure 44 can be usedto detect alignment with the coil inside the neurostimulator device 2 ofFIG. 1. Thus, the coil assembly structure 44 may also be referred to asa coil positioning system (CPS). When the coil assembly structure 44 andthe coil of the neurostimulator device 2 are aligned, transcutaneousenergy transfer is optimized. The coil assembly structure 44 and thealignment detection process will be discussed in more detail below.

The position-sensing circuitry 46 includes a plurality of active and/orpassive components, such as transistors, resistors, capacitors, andinductors. The position-sensing circuitry 46 is operable to correlate aphysical displacement or movement of the coil assembly structure 44relative to the internal coil of the neurostimulator device 2 (ofFIG. 1) by measuring electrical characteristics associated with the coilassembly structure 44, for example a voltage in one of the secondaryposition-sensing coils. As such, the position-sensing circuitry 46 canbe used to ascertain locational information of the coil assemblystructure 44. Such locational information can be processed by theprocessor and additional circuitry 47 and communicated to a user throughthe communication interface 48. In an embodiment, the communicationinterface 48 includes a visual display unit, such as a flat panelscreen. In other embodiments, the communication interface 48 may includeother means of communication, such as one or more audio or tactilecomponents. For example, the communication interface 48 may audiblyinstruct an operator where or how to move the coil assembly structure 44in order to achieve optimum alignment. Based on the informationcommunicated through the communication interface 48, an operator canadjust the alignment between the coil assembly structure 44 with theinternal coil of the neurostimulator device 2 until the alignment isoptimum for transcutaneous energy transfer. The position-sensingcircuitry 46 and the communication interface 48 will be discussed inmore detail below.

FIG. 3 illustrates a simplified fragmentary cross-sectional side view ofa transcutaneous energy transfer device (TET device) 100, a coilassembly 101 coupled to the TET device 100, and an implantable pulsegenerator (IPG) 150. The TET device 100 is implemented as an embodimentof the external charging device 40 shown in FIG. 2, the coil assembly101 is implemented as an embodiment of the coil assembly structure 44 ofFIG. 2, and the IPG 150 is implemented as an embodiment of theneurostimulator device 2 shown in FIG. 1. Those of ordinary skill in theart will understand, however, that the IPG 150 is not limited to aneurostimulator. In other embodiments, the IPG 150 may be a differenttype of IPG, including, for example, a pacemaker, a defibrillator, atrial stimulator or any other type of medical device.

As discussed above, several rechargeable medical devices implanted inthe human body require power (or energy) to be supplied transcutaneouslyfrom a TET device to the implantable medical device. This energytransfer is typically provided via an inductive link consisting of anexternal transmitter coil that generates an alternating electromagneticfield and a receiver coil in the implant that converts the receivedelectromagnetic energy into electrical energy to generate power. Thepower that is delivered over the transcutaneous link is used by theimplant to power its internal electronics, recharge a battery, or both.The IPG 150 herein is structurally configured and arranged for wirelessprogramming and control through the skin of the patient. Accordingly, itmay include a transceiver (e.g., transceiver 4 of FIG. 1) capable ofcommunicating with external programming and control devices, such as theTET device 100 and a human operator. It also includes a rechargeablepower source (e.g., power source 10 of FIG. 1) that can be configured tobe wirelessly recharged through the patient's skin when the coilassembly 101 connected to the TET device 100 is externally placed in theproximity of the IPG 150.

In the embodiment shown in FIG. 3, the IPG includes an IPG charging coil154 used to recharge the IPG's rechargeable power source. Those ofordinary skill in the art will recognize that other components may alsobe included within an IPG, such as a transceiver to transmit/receivedata to/from an external controller, sensors, pulse generationcircuitry, microcontrollers, power conditioning circuitry, andprotection circuitry.

The IPG charging coil 154 can receive power while the IPG 150 isimplanted in a patient through the use of the TET device 100. Therechargeable power source within the IPG 150 may be any of a variety ofpower sources including a chemically-based battery or a capacitor. Therechargeable power source may also be a well known lithium ion battery.The TET coil 104 induces in the position-sensing coils 106, 108 anoscillating or alternating current while also inducing current in thecharging coil 154 when the TET coil 104 is placed in the proximity ofthe charging coil 154.

As shown in FIG. 3, the TET device 100 and the coil assembly 101 arelocated outside the patient in which the IPG 150 is implanted. The coilassembly 101 is positioned above but near the external surface of theskin S, slightly lateral to the placement of the IPG 150, which is shownimplanted beneath the skin S of the patient. Although the TET device 100is shown with a rectangular configuration, it should be understood thatthe TET device 100 may take any desired shape. Power is provided to theTET device 100 through a rechargeable battery 102. In the illustratedembodiment, a transcutaneous energy transfer coil (hereinafter “TETcoil”) 104 and two position-sensing coils 106 and 108 are disposedtogether within the coil assembly 101. A circuitry section 109correlates electrical characteristics of the coil assembly 101 with anymovement or physical displacement of the coil assembly 101 relative tothe IPG 150. After computing the locational information of the coilassembly 101, the circuitry section 109 (which includes position-sensingcircuitry) instructs a display screen 110 to communicate such locationalinformation. The display screen 110 is disposed on the surface 112 ofthe TET device 100. In the embodiment shown, the display screen 110 canexhibit a visual representation of the alignment or relative dispositionof the and the IPG 150 to an operator.

FIGS. 4 and 5 are simplified top views of the TET device 100, the coilassembly 101 that is positioned slightly above the skin, and an IPG 150implanted subcutaneously under a patient's skin. Efficient energytransfer is dependent on the proper co-axial alignment of the TET coil104 (of the coil assembly 101) with the IPG's charging coil 154. FIG. 4illustrates the coil assembly 101 being moved in a plane parallel to theskin surface towards the IPG 150. Implantable medical devices such asthe illustrated IPG 150 are typically implanted subcutaneously atdepths, depending upon the type of device, ranging from 0.5 centimeterto 2.5 centimeters. After the IPG 150 has been surgically implanted inthe patient, the location of the implant is often discernable by avisual and/or tactile sign such as a scar, an indentation of the skin,or a bulging area of skin. Nevertheless, the precise position of thecharging coil 154 within the IPG 150 is difficult to discern using onlyvisual or tactile signs. Exacerbating the problem is the fact that theIPG coil 154 may not be centered within the IPG 150. In such a case,even if the TET coil 104 is properly aligned over the IPG 150, the TETcoil 104 and the IPG coil 154 may still not be precisely aligned. Thoughthe operator of the TET device 100 may use the visual or tactile signsof implantation to approximate the location of the IPG 150, precisealignment of the TET coil 104 and IPG coil 154 can be difficult.

FIG. 5 illustrates the coil assembly 101 substantially aligned with theIPG coil 154. When the coil assembly 101 (the TET coil 104, inparticular) is centered over the IPG coil 154, the efficiency of thetranscutaneous energy transfer between the TET coil 104 and the IPG coil154 is maximized. Thus, the best performance of energy transfer isobtained when the two coils, the TET coil 104 and the IPG coil 154, arealigned as shown in FIG. 5, but this alignment is not easy to achievebecause the exact position of the IPG coil 154 is often difficult toidentify. Any misalignment between the two coils reduces the efficiencyof the energy transfer because a smaller quantity of energy is beingreceived by the IPG coil 154, resulting in the TET coil 104 needing todeliver more power to complete the recharging process, and potentiallymultiple charging sessions to achieve a full recharge.

For implanted devices, the efficiency at which energy istranscutaneously transferred is important. The inductive coupling of theTET coil 104 and the IPG coil 154 has a tendency to heat the surroundingtissue. Heat generated by the coil assembly 101 and the IPG 150 is lostinto the surrounding tissue, causing an increase in the temperature ofthe surrounding tissue. Depending on the time required for the energytransfer, such a temperature increase may result in side effects rangingfrom mere discomfort to severe tissue lesions. The higher the efficiencyof energy transfer, the more energy can be transferred while at the sametime limiting the heating of surrounding tissue. The present disclosuremay be used to facilitate the efficient alignment and charging of anycooperating pair of TET device 100 and implantable medical device. Forexample, the present TET device 100 could be used as a part of a systememploying an implantable medical device such as a cardiac pacemaker,cardioverter, defibrillator, an implantable drug pump, or a nervestimulator.

FIG. 6A-6B are cut-away top views of a coil assembly 160 comprising aTET coil 104 and two position-sensing coils 106 and 108. For the sake ofvisual clarity, the position-sensing coils 106 and 108 are shownseparately, with the coil 106 being shown in FIG. 6A, and the coil 108being shown in FIG. 6B. In an embodiment, the position-sensing coils 106and 108 are substantially identical in size and shape and are situatedsuch that the coils are concentric and perpendicularly oriented witheach other. For example, the position-sensing coil 106 is oriented in anX-direction or an X-axis, and the position-sensing coil 108 is orientedin a Y-direction or a Y-axis. The position-sensing coils 106, 108 areeach configured in a “Double D” winding shape, and are disposed in amanner such that they are centered against and overlaying the TET coil104. It is envisioned that other sense coil arrangements can also beemployed to provide alignment feedback.

In the present embodiment, the TET coil 104 is a three layer spiralcomprising 13/12/10 turns of Litz wire 65/36. The TET coil 104 is showncircumferentially disposed around a toroid core 170. The toroid core 170may comprise a ferrite core which increases the magnetic flux density tofacilitate the energy transfer between the TET coil 104 and the IPG coil154. Stated differently, the toroid core 170 helps to focus theelectromagnetic energy generated by the TET coil 104 transcutaneouslytoward the IPG coil 154. Alternatively, the toroid core 170 couldcomprise other structures or materials, or could represent an air core.

FIG. 6C is a cross-sectional view of the coil assembly 160 of FIG. 6Ataken from point A to point A′. The three layers of the TET coil 104,the toroid core 170, and a TET coil substrate 172 are diagrammed in FIG.6C. The TET coil substrate 172 directly abuts and underlies the flatcircular surface of the TET coil 104 that is configured to face the IPG150. The TET coil substrate 172 contains a printed circuit board (PCB)with printed sense coils. In addition, the TET coil 104 could beintegrated with or wound around other electronic components in the TETdevice 100.

The coil assembly 160 cooperates with the IPG 150 to detect optimumalignment of the TET coil 104 and the IPG coil 154. The TET coil 104 andthe position-sensing coils 106 and 108 together function in a similarmanner to a linear variable differential transformer (hereinafter“LVDT”), which can be used to measure linear displacement. An exampleLVDT has three solenoidal coils placed around a tube. The center coil isthe primary coil and the two outer coils are the secondary coils. In anembodiment, the outer secondary coils are connected in reverse series. Acylindrical ferromagnetic core is attached to an object whose positionis to be measured, and the core slides along the axis of the tube. Analternating current is driven though the primary coil, which induces avoltage in each secondary coil in proportion to its mutual inductancewith the primary coil. As the core moves along the tube, these mutualinductances change, causing the voltages induced in the secondary coilsto change as well. Because the coils are connected in reverse series,the output voltage is the difference between the two secondary voltages.When the core is equidistant between the two secondary coils, equal butopposite voltages are induced in both secondary coils and the outputvoltage is zero. If the core is displaced in one direction, the voltagein one coil increases as the other decreases, causing the output voltageto increase proportionally to the degree of displacement to one side. Ifthe core moves in the other direction, the voltage will also increase,but in a polarity opposite to that of the primary voltage.

In the present embodiment, the TET coil 104 serves as the primary coilof the LVDT. The “double D” position-sensing coils 106, 108 serve as thesecondary coils. The position-sensing coil 106 recognizes displacementalong the X-axis, and the position-sensing coil 108 recognizesdisplacement along the Y-axis. Instead of a ferromagnetic core, thepresent embodiment uses the IPG's coil 154 to shape the magnetic fieldaround the TET coil 104. The position-sensing coils 106, 108 aremechanically aligned with the TET coil 104 such that equal and oppositevoltages are induced in the position-sensing coils 106, 108 when thecoil assembly 160 is centered in the corresponding axis upon the IPGcoil 154. As the coil assembly 160 moves off center so that IPG coil 154is closer to one “D” of the position-sensing coil 106 (or 108) andfarther from the other, a net AC voltage is induced in thatposition-sensing coil. The voltage increases in proportion to thedisplacement of the IPG coil 154 from the center of the position-sensingcoil 106 (or 108). This voltage reaches a maximum when the displacementapproaches the outer circumference of the TET coil 104, and returns tozero for larger displacements.

FIGS. 7A and 7B are graphical representations of a set of outputvoltages from the position sense circuitry 46 of FIG. 2 vs. thetranscutaneous energy transfer coil displacement. Rectification of theposition-sensing coil 106 (or 108) “double-D” output (driven by the TETcoil 104) creates an S-curve 174 relationship between coil displacementin one axis and rectifier output voltage. The voltage is eitherpositive, negative, or zero depending upon the direction and magnitudeof the coil displacement. FIG. 7A represents the voltage relative to thedisplacement in the X-axis, and FIG. 7B represents the voltage relativeto the displacement in the Y-axis. As mentioned above, theposition-sensing coil 106 recognizes displacement along the X-axis, andthe position-sensing coil 108 recognizes displacement along the Y-axis.The dual output of the pair of perpendicularly-oriented position-sensingcoils 106, 108 allows the coil assembly 160 to recognize and reflectaxial alignment.

Using the S-curves 174 alone to determine coil position and alignmentmay be inaccurate due to the fact that the output voltage will return tozero as the displacement moves beyond the radius of the position-sensingcoils 106, 108 (i.e., when the IPG charging coil 154 is laterallydisplaced beyond the outer circumference of the position-sensing coils106, 108). Such large displacements in either the X-axis or Y-axisdirection produce a zero voltage rectifier output and may falselyindicate optimal TET coil 104 to IPG coil 154 alignment. This scenariois remedied by circuitry that looks for a zero-crossing event. When azero-crossing event is detected, a flip-flop is set indicating optimalTET coil 104 to IPG coil 154 alignment in a single axis. A windowcomparator resets the flip-flop if the voltage moves outside plus/minuslimits about zero.

FIG. 8 is a schematic block diagram illustrating a portion of theelectrical circuitry (e.g., position-sensing circuitry 46 of FIG. 2 orelectrical circuitry 109 of FIG. 3) of the coil positioning system. Eachof the double-D IPG positioning coils 106, 108 are processed byidentical circuits. For this reason only one of such circuit is shown inFIG. 8.

The block diagram shown in FIG. 8 includes a preamplifier section 210, arectifier section 215, a filter section 220, a comparators section 225,and an analog-to-digital converter section 230. The preamplifier section210 includes circuitry for amplifying a signal from one of theposition-sensing coils 106, 108 (shown in FIGS. 6A-6B), which is shownhere as sense coil input component 240. The rectifier section 215 iscoupled to the preamplifier section 210 and includes circuitry forconverting AC components of the output of the preamplifier section 210into DC components. The filter section 220 is coupled to the rectifiersection 215 and includes circuitry for filtering an output of therectifier section 215. The comparators section 225 is coupled to thefilter section 220 and includes circuitry for, among other things,detecting a zero-crossing event. An analog-to-digital converter section230 is also coupled to the filter section 220 and is used to provide adigital signal representing the coil assembly 101 displacement in oneaxis. The output of the analog-to-digital converter section 230 isprocessed by the processor 47, which would in turn provide instructionto the user via the communication interface 48.

FIGS. 9A and 9B are graphical views of an embodiment of the TET device100 in accordance with the present disclosure. As shown in thesefigures, the TET device 100 comprises a handheld battery-operated devicethat communicates with the IPG 150. The TET device 100 comprises acompact outer housing 278 sized to fit comfortably in the operator'shand. In the illustrated embodiment, the housing 278 is apolyhedron-shaped container configured to contain the various componentsof the TET device 100. The housing 278 may be alternatively configuredto conform to the operator's palm or be configured in a variety of othershapes. In one example, the housing 278 forming the TET device 100 has athickness of less than about 1.5 inch, a width of less than about 3inches, and a height of less than about 4 inches. However, both largerand smaller sizes are contemplated. To minimize weight and maximize easeof maneuverability, the housing 278 is constructed of a durable andlightweight material such as plastic. The TET device 100 operates usingreplaceable and/or rechargeable batteries.

The TET device 100 comprises a flat panel display screen 110. In theillustrated embodiment, the display screen 110 comprises a liquidcrystal display (“LCD”) (or another suitable portable display) anddisplays coil alignment when this mode is activated. Alternativeembodiments may include buttons that allow the IPG 150 to be turned ONor OFF, provide for the adjustment of various alignment parameters, orprovide for the selection of different display screens. Some functionsor screens may be accessible through the repeated pushing of particularbuttons, pushing various buttons in combination or in a certainsequence, or by pushing buttons for an extended period of time.

In the embodiment shown, the display screen 110 is designed to conveyinformation to the operator about the alignment of the coil assembly 101to the IPG 150. FIG. 9A illustrates the alignment image of the displayscreen 110 a when the coil assembly 101 and the IPG 150 are not inalignment, and FIG. 9B illustrates the alignment image on the displayscreen 110 b when the coil assembly 101 and the IPG 150 are inalignment. When the TET device 100 is turned on and the coil assembly101 is moved into proximity with the IPG 150, a representative IPG image282 is shown fixed in the center of the screen 110. As the operatormoves the coil assembly 101 into proximity with the IPG 150, crosshairs283 (visually representing the coil assembly 101) will appear on thedisplay screen 110 and move around and over the fixed IPG image 282 in avisual representation of the alignment status of the TET coil 104 andthe IPG charging coil 154. The distance of the onscreen crosshairs 283(representing the TET coil 104) from the onscreen IPG image 282(representing the IPG coil 154) in each of the mutually perpendicularaxes is correlated to the voltage output of each of the rectifiedposition-sensing coils 106, 108. For example, this correlation may becalculated by the circuitry section 47 shown in FIG. 2, where voltagesmeasured from the position sense circuitry 46 are converted into digitalsignals that can be analyzed by the processor 47. The processor 47 iscapable of driving the communications interface 48, which in thisembodiment is a display screen 110, with a variety of inputs and/orcommands. Optimal coil alignment is indicated on the display screen 110b, in FIG. 9B, when the crosshairs 283 are shown centered on anddirectly superimposed over the representative IPG image 282.

In an embodiment, to achieve optimal coil alignment, the display screen110 may display visual instructions such as text instructions telling anoperator where or how to move the TET coil assembly 101. For example,the text instructions may be, “move the charging device to the right”(or in another suitable direction). The text instructions may beconstantly updated until optimum alignment is achieved between the coilassembly 101 and the IPG 150. At that point, the display 110 may displaya text indicating optimum alignment has been achieved, and no moremovement is necessary. It is understood that in some embodiments, themovement directions may be audible in nature, or may be a combination ofaudible and visual.

Based on the discussions above, it can be seen that the display 110advantageously allows the operator to receive real-time visual feedbackindicating not only the quality of the coil alignment, but also in whichdirection they should move the coil assembly 101 to achieve optimal coilalignment. Other types of alignment displays are also contemplated.

FIGS. 10 and 11 each contain a portion of a flow chart illustrating anexemplary alignment and charging of an implantable medical device usinga TET device according to embodiments of the present disclosure. Withinitial reference to FIG. 10, the TET device 100 is powered up at thestart step 300. At step 302, the TET device 100 determines its currentcharge, and at step 304, the TET device 100 displays that charge statusof the TET device battery. At step 306, the TET device 100 determineswhether the charge is sufficient to proceed with charging of the IPG150. If the TET device 100 charge is deemed insufficient, then the TETdevice 100 continues to be powered up at step 300 and the chargecontinues to be determined and displayed at steps 302 and 304,respectively, until the TET device 100 charge is deemed sufficient atstep 306. Once the TET device 100 charge is deemed to be sufficient,then the operator may position the coil assembly 101 over the skin inproximity with the IPG 150 in step 308. The operator may be guided inthis initial placement by visual or tactile signs of implantation, suchas a scar, an indentation of the skin, or a bulging area of skin.

When the coil assembly 101 is in proximity with the IPG 150, the displayscreen 110 displays crosshairs 283 and a representative IPG image 282 instep 310. The representative IPG image 282 remains fixed in the centerof the display while the crosshairs 283 (representative of the TET coil104) move in relation to the degree of lateral displacement of the TETcoil 104 from the IPG coil 154. At step 312, the operator must ascertainwhether the crosshairs 283 are displayed on the display screen 110. Ifthe crosshairs 283 are not visible, this indicates that the coilassembly 101 is not positioned in proximity with the IPG 150 and theoperator must then reposition the coil assembly 101 in proximity withthe IPG 150. If the crosshairs 283 are visible, the coil assembly 101 islocated in proximity with the IPG 150 and the operator may then move thecoil assembly 101 until the crosshairs 283 are displayed as centered onand superimposed over the representative IPG image 282 in steps 314 and316. At step 318, the operator must evaluate whether the crosshairs 283are centered on over the representative IPG image 282. If the operatordoes not see the crosshairs 283 centered on the representative IPG image282, then the operator must return to step 314 and move the coilassembly 101 until the crosshairs 283 are displayed as centered on andsuperimposed over the representative IPG image 282 in step 316.

Proceeding to FIG. 11, if the operator determines that the crosshairs283 are centered over the representative IPG image 282, then the TETdevice 100 will simultaneously determine the existing charge status ofthe IPG 150 at step 320 and determine the electromagnetic field strengthgenerated by the coil assembly 101 and the IPG charging coil 154 at step322. The TET device 100 then displays the existing charge status of theIPG in step 324 and displays the electromagnetic field strength in step326. In step 328, the TET device 100 determines the remaining IPG chargetime, then displays the remaining IPG charge time in step 330. At step332, the TET device 100 must evaluate whether the IPG 150 is completelycharged. If the IPG 150 is not completely charged, the TET device 100must continue to charge the IPG 150 with the coil assembly 101 heldstationary above the IPG 150. If the IPG 150 is completely charged, theTET device 100 will stop charging the IPG 150 and the operator mayremove the coil assembly 101 and power down the TET device 100 at step334.

FIG. 12 is a flowchart illustrating an operational method 500 of the TETdevice described above. The method 500 includes block 510, which a coilassembly is provided. The coil assembly includes a primary coil and aplurality of position-sensing coils. The method 500 includes block 520in which an electrical parameter of the coil assembly is measured. Themethod 500 includes block 530 in which positional information of thecoil assembly is determined relative to the implanted medical device.The method 500 includes block 540 in which the positional information iscommunicated. The communication may be done through a communicationinterface, which may include a visual display.

FIG. 13A is a side view of a spine 1000, and FIG. 13B is a posteriorview of the spine 1000. The spine 1000 includes a cervical region 1010,a thoracic region 1020, a lumbar region 1030, and a sacrococcygealregion 1040. The cervical region 1010 includes the top 7 vertebrae,which may be designated with C1-C7. The thoracic region 1020 includesthe next 12 vertebrae below the cervical region 1010, which may bedesignated with T1-T12. The lumbar region 1030 includes the final 5“true” vertebrae, which may be designated with L1-L5. The sacrococcygealregion 1040 includes 9 fused vertebrae that make up the sacrum and thecoccyx. The fused vertebrae of the sacrum may be designated with S1-S5.

Neural tissue (not illustrated for the sake of simplicity) branch offfrom the spinal cord through spaces between the vertebrae. The neuraltissue can be individually and selectively stimulated in accordance withvarious aspects of the present disclosure. For example, referring toFIG. 13B, an IPG device 1100 is shown implanted inside the body andcommunicating with a TET device 1200. The IPG device 1100 may includevarious embodiments of the IPG 150 described above. The TET device 1200may include various embodiments of the TET device 100 described above. Aconductive lead 1110 is electrically coupled to the circuitry inside theIPG device 1100. The conductive lead 1110 may be removably coupled tothe IPG device 1100 through a connector, for example. A distal end ofthe conductive lead 1110 is attached to one or more electrodes 1120. Theelectrodes 1120 are implanted adjacent to a desired nerve tissue in thethoracic region 1020. The distal end of the lead 1110 with itsaccompanying electrodes may be positioned beneath the dura mater usingwell-established and known techniques in the art.

The electrodes 1120 deliver current drawn from the IPG device 1100,therefore generating an electric field near the neural tissue. Theelectric field stimulates the neural tissue to accomplish its intendedfunctions. For example, the neural stimulation may alleviate pain in anembodiment. In other embodiments, a stimulator as described above may beplaced in different locations throughout the body and may be programmedto address a variety of problems, including for example but withoutlimitation: prevention or reduction of epileptic seizures, weightcontrol or regulation of heart beats.

It is understood that the IPG 1100, the lead 1110, and the electrodes1120 may be implanted completely inside the body, may be positionedcompletely outside the body or may have only one or more componentsimplanted within the body while other components remain outside thebody. When they are implanted inside the body, the implant location maybe adjusted (e.g., anywhere along the spine 1000) to deliver theintended therapeutic effects of spinal cord electrical stimulation in adesired region of the spine. As is apparent in FIG. 13B, the IPG 1100may be able to communicate with and be charged by the TET device 1200.

The devices, systems, and methods described herein provide an improvedand more accurate system of positioning a TET device over and chargingan IPG implanted in a patient by providing the operator with a visualindication of the direction in which the TET device needs to be moved inorder to obtain the perfect alignment between a TET coil and an IPGcharging coil. The TET device as described herein also enables aconstant monitoring of the efficiency of the energy transfer from theTET coil and the IPG charging coil.

Applicants note that the procedures disclosed herein are merelyexemplary and that the systems and methods disclosed herein may beutilized for numerous other medical processes and procedures. Althoughseveral selected embodiments have been illustrated and described indetail, it will be understood that they are exemplary, and that avariety of substitutions and alterations are possible without departingfrom the spirit and scope of the present disclosure, as defined by thefollowing claims.

What is claimed is:
 1. A charger for an implanted medical device, thecharger comprising: a power supply; a coil assembly electrically coupledto the power supply, the coil assembly including a primary coil and aplurality of sense coils, wherein at least two of the sense coils areperpendicularly oriented with respect to one another; electricalcircuitry configured to: measure electrical parameters of the coilassembly; and determine, based on the measured electrical parameters,locational information with respect to the coil assembly and theimplanted medical device; and a visual interface configured to visuallycommunicate the locational information.
 2. The charger of claim 1,wherein the sense coils are located in different layers of a substrate.3. The charger of claim 1, wherein the primary coil is located over thesense coils.
 4. The charger of claim 1, wherein the electricalparameters comprise voltages of one or more of the sense coils.
 5. Thecharger of claim 1, wherein the electrical circuitry is configured tocalculate, based on the measured electrical parameters, a locationaloffset between the coil assembly and the implanted medical device. 6.The charger of claim 1, wherein the electrical circuitry comprises:amplification circuitry configured to amplify electrical signals fromthe sense coils; rectification circuitry configured to convert ACcomponents of an output of the amplification circuitry into DCcomponents; and filtering circuitry configured to filter an output ofthe rectification circuitry.
 7. The charger of claim 6, wherein theelectrical circuitry further comprises comparator circuitry coupled tothe filtering circuitry, the comparator circuitry being configured todetect a zero-crossing event.
 8. The charger of claim 1, wherein thevisual interface is configured to display, on a screen, a first visualobject representing a location of the coil assembly and a second visualobject representing a location of the implanted medical device.
 9. Thecharger of claim 8, wherein the visual interface is configured to updatethe locations of the coil assembly and the implanted medical device inreal time in response to actual movements of the coil assembly or theimplanted medical device.
 10. The charger of claim 1, wherein the visualinterface is configured to communicate instructions to a user to alignthe coil assembly with the implanted medical device.
 11. A method forcharging an implanted medical device, comprising: providing a coilassembly, the coil assembly including a primary coil and a plurality ofsense coils, wherein at least two of the sense coils are perpendicularlyoriented with respect to one another; measuring electrical parameters ofthe coil assembly; determining, based on the measured electricalparameters, locational information with respect to the coil assembly andthe implanted medical device; and communicating the locationalinformation via a visual interface.
 12. The method of claim 11, whereinthe sense coils are located in different layers of a substrate, andwherein the primary coil is located over the sense coils.
 13. The methodof claim 11, wherein the measuring of the electrical parameterscomprises measuring voltages of one or more of the sense coils.
 14. Themethod of claim 11, wherein the determining comprises calculating, basedon the measured electrical parameters, a locational offset between thecoil assembly and the implanted medical device.
 15. The method of claim11, wherein the determining comprises: amplifying electrical signalsfrom the sense coils; converting AC components of the amplifiedelectrical signals into DC components; filtering the DC components ofthe amplified electrical signals; and detecting a zero-crossing eventbased on the filtered DC components.
 16. The method of claim 11, whereinthe communicating comprises displaying, on a screen, a first visualobject representing a location of the coil assembly and a second visualobject representing a location of the implanted medical device.
 17. Themethod of claim 11, wherein the communicating comprises communicatinginstructions to a user to align the coil assembly with the implantedmedical device.
 18. A medical system, comprising: an implantable medicaldevice; and a charger for charging the implantable medical device,wherein the charger includes: a coil assembly, circuitry that iselectrically coupled to the coil assembly, and a communicationsinterface, wherein: the coil assembly includes: a primary coil; a firstposition-sensing coil; and a second position-sensing coil that isperpendicularly oriented with respect to the first position-sensingcoil; the circuitry is configured to: measure voltages of the first andsecond position-sensing coils; and determine, based on the measuredvoltages, locational information with respect to the coil assembly andthe implantable medical device; and the communications interface isconfigured to visually communicate the locational information.
 19. Themedical charging system of claim 18, wherein the circuitry comprises:amplification circuitry configured to amplify electrical signals fromthe sense coils; rectification circuitry configured to covert ACcomponents of an output of the amplification circuitry into DCcomponents; filtering circuitry configured to filter an output of therectification circuitry; and comparator circuitry coupled to thefiltering circuitry, the comparator circuitry being configured to detecta zero-crossing event based on the filtered output of the rectificationcircuitry.
 20. The medical charging system of claim 18, wherein thecommunications interface is configured to display, in real time, a firstvisual object representing a location of the coil assembly and a secondvisual object representing a location of the implantable medical device.